As the industry gradually advances toward fineness, speedup and mass production, it is required to improve cutting performance and life cycle of a cutting tool. Especially, since high heat of 900° C. or more is locally generated on a front end of the cutting tool rubbing with a workpiece in a rapid cutting of a workpiece having a high hardness and a cutting of a difficult-to-cut material having a low thermal conductivity, life cycle of the cutting tool may be improved by forming a hard film having excellent oxidation resistance and abrasion resistance on a cutting surface of the cutting tool.
In order to improve the cutting performance and life cycle, a single-layered hard film including TiN, Al2O3, TiAlN, AlTiN, AlCrN or the like having abrasion resistance, oxidation resistance, impact resistance and the like, or a multi-layered hard film in which the single-layered hard films are stacked in two layers or more, is formed on a hard base material such as a hard metal, a cermet, a high speed steel, an end mill, a drill, or the like to cope with demands for the high hardness workpiece and the difficult-to-cut material.
Recently, the hardness of the workpiece is gradually increased, and a demand for processing of a difficult-to-cut material having a low thermal conductivity and severely fused on a tool is also increased. Especially, since stainless steel, heat-resistant alloy steel, ductile cast iron and the like have a low thermal conductance compared to general steels, cutting heat is not emitted due to chips in a cutting and heat is concentrated on a cutting edge portion of the cutting tool, abrasion, seizure and exfoliation phenomena are easily generated on the cutting edge of the cutting tool due to a chemical reaction between the cutting tool and the workpiece, and life cycle of the cutting tool is rapidly reduced.
Therefore, only with the single layered or multi-layered structures having a composition described above, it becomes more and more difficult to cope with a demand for a cutting tool for the cutting of such a difficult-to-be cut material and ductile cast iron, which are required to evenly have characteristics such as excellent abrasion resistance, oxidation resistance and lubrication.
Therefore, trials improving the cutting performance are recently increased through a method for regularly and repeatedly stacking at least two thin films having a nano level that are different in material property.
For example, Korea Patent No. 876366 discloses a thin film structure in which a lower layer is deposited on an insert, an end mill, a drill or a cermet which is a hard metal tool through a physical vapor deposition, in order to improve an adhesion force and align the orientation of crystal grains in the direction of (200) plane, a (Ti,Al)N multi-layered thin film which is a middle layer is continuously deposited in order to improve impact resistance and chipping resistance, and a top layer which is comprised of TiAlN or AlTiSiN, is constituted by layer A, layer B, layer C and layer D and has a structure in which layer A, layer B, layer C and layer D are alternately stacked, is formed to improve the abrasion resistance and oxidation resistance of the top layer.
While the abrasion resistance and oxidation resistance may be improved through the multi-layered structure as described above, development of a hard film having a novel structure is required in order to evenly improve various characteristics such as abrasion resistance, impact resistance (toughness) and chipping resistance required for the cutting